Among fundamentals of basic concepts of communication theory, a few things necessary for a description of the present invention will now be explained.
In this document, the new system indicates a communication system which is designed to support the legacy system, especially to support the mobile station (MS) of the legacy system. Therefore, it can be the cases that the MSs of the new system and the MSs of the legacy system communicate with the base station of the new system. For example, the base station of IEEE Standard 802.16m (hereinafter ‘16 m’) system may have to support at least one MS of the 16m system and at least one MS of IEEE Standard 802.16e (hereinafter ‘16e’) system in a single frame.
In telecommunications and computer network, multiplexing (known as muxing) is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several phone calls may be transferred using one wire.
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side.
The two most basic forms of multiplexing are time-division multiplexing (TDM) and frequency-division multiplexing (FDM), both either in analog or digital form. FDM requires modulation of each signal.
An uplink (UL or U/L) is the portion of a communication link used for the transmission of signals. Pertaining to GSM (Global System for Mobile communications) and cellular networks, the radio uplink is the transmission path from the Mobile Station (Cell Phone) to a Base Station (Cell Site). An uplink is the inverse of a downlink. Pertaining to cellular networks, the radio downlink is the transmission path from a Base Transceiver Station (Cell Site) to the Mobile Station (Cell Phone).
At least on of TDM and FDM can be taken into consideration when multiplexing MSs of the new system and MSs of the legacy system in a single frame.
For an uplink communication, FDM is advantageous for a MS of the legacy system in that cell coverage of the legacy system and throughput of a cell-edge located user of the legacy system are maintained, whereas TDM has an advantage that the design flexibility of the new system can be maximized.
A cellular network is a radio network made up of a number of radio cells (or just cells) each served by a fixed transmitter, known as a cell site or base station. These cells are used to cover different areas in order to provide radio coverage over a wider area than the area of one cell. Cellular networks are inherently asymmetric with a set of fixed main transceivers each serving a cell and a set of distributed (generally, but not always, mobile) transceivers which provide services to the network's users.
Cellular networks offer a number of advantages in terms of capacity, power usage, and coverage over alternative solutions.
The increased capacity in a cellular network, compared with a network with a single transmitter, comes from the fact that the same radio frequency can be reused in a different area for a completely different transmission. If there is a single plain transmitter, only one transmission can be used on any given frequency. Unfortunately, there is inevitably some level of interference from the signal from the other cells which use the same frequency. This means that, in a standard FDMA system, there must be at least a one cell gap between cells which reuse the same frequency.
The frequency reuse factor is the rate at which the same frequency can be used in the network. It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies for transmission. Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation).
In case of N sector antennas on the same base station site, each with different direction, the base station site can serve N different sectors. N is typically 3. A reuse pattern of N/K denotes a further division in frequency among N sector antennas per site. Some current and historical reuse patterns are 3/7 (North American AMPS), 6/4 (Motorola NAMPS), and 3/4 (GSM).
If the total available bandwidth is B, each cell can only utilize a number of frequency channels corresponding to a bandwidth of B/K, and each sector can use a bandwidth of B/NK.
For a multi-cell configuration, each cell may adopt different multiplexing scheme for the same time frame, that is, one cell may use TDM for a frame and another cell adjacent to the one cell may use FDM for the same frame. In this case, inter-cell-interference may occur such that system performance decreases. Therefore, it is necessary to take inter-cell-interference into consideration when designing multiplexing of the MSs of the new system and the MSs of the legacy system.
In addition, a BS in a cell may need to broadcast, for each frame, information about which one among TDM and FDM is used for uplink transmission to the MSs located in the cell. The MSs may use the broadcast information for subcarrier mapping.